There have been many attempts at producing machines capable of recognising speech. Nearly all have relied on some torm of feature extraction technique followed by pattern matching using techniques such as dynamic programming. In this paper we introduce PROLOG as a language for the labelling of vowels in continuous speech. The recogniser stores descriptions of vowels which are then compared with the incoming data, where a label of a phoneme is assigned together with a value for its membership of a particular vowel class. The vowel recogniser proposed has been designed to be used together with higher level linguistic knowledge to torm the basis of a speech recognition system. The use of differing membership functions is examined.